(1) Field of the Invention
This invention relates to a pleat type cartridge filter.
(2) Description of the Prior Art
Various ultramicro-porous membrane filters in which each pore is no more than 5 .mu. are used for microfiltration or ultrafiltration of air, gas, liquid, etc. Today, the demand for such ultramicro-porous membrane filters is increased more and more due to rapid progress of electronics, biotechnology and other scientific field.
There are known various kinds of cartridge filters using the membrane made of cellulose, polyamide, polyester, polypropylene, polyethylene, nonwoven material of glass fiber, paper or the like.
This invention provides further improvements of such conventional cartridge filters as well as a novel cartridge filter whose all components are made of polytetrafluoroethylene (hereinafter called "TFE"). Because TFE has excellent physical and chemical properties, it has been expected to produce all TFE-made cartridge filter, but so far it has been considered to be impossible to produce such a cartridge filter.
An average diameter of a conventional cartridge filter is about 75 mm. This invention enables production of a large-size cartridge filter having more than 500 mm in diameter.
Before describing preferred examples of my invention, I refer to some problems of a TFE-made cartridge filter.
For the purpose of realizing accurate microfiltration, filter membranes must have excellent properties such as high temperature resistance, high chemical resistance, high tensile resistance, etc. To satisfy those requirements, all components of the cartridge filter such as membrane, support material, core, end cap, protective cage, etc. must also have such high performance. It is no exaggeration to say that materials capable of reflecting such a high performance will be limited to TFE and other few fluorinated polymers.
The following membranes are known: elongated membranes as disclosed in Japanese Examined Patent Publication No. 42-13560, published August 1976, Japanese Unexamined Patent Publication No. 49-118761, published November 1974, Japanese Examined Patent Publication No. 51-18991, published June 1976, or membranes as disclosed in Japanese Patent Publication No. 60-97832, published May 1985 or microporous membranes made of alkoxyethylene fluoride polymer (PFA), ethylene-propylene-ether copolymer fluoride (EPA) and tetrafluoroethylene-6-fluoropolypropylene copolymer (FEP) as described in Japanese Patent Application No. 59-135696 (filed in June 1984 in the name of the present inventor).
The TFE-made elongated membrane which is being used practically is so thin and flexible that it is not applicable for a conventional cartridge structure. A conventional membrane is supported on its surface by a supporting membrane and folded in a pleat form. Both upper and lower ends of the TFE-made membrane are sealed by heating means and jointed to both end caps by melting means. Since the TFE membrane is an elongated membrane, it is rapidly shrinked at a preheating time, i.e. when it begins to be softened. As a result, it becomes impossible to seal the TFE membrane.
Even if the end caps are made of PFA or EPE, those fluorinated polymers has smaller intermolecular affinity, melting means between an end of the TFE membrane and such end cap requires a certain pressure, so that it is very difficult to apply a certain pressure for a melted surface, thereby a complete sealing therebetween is not attainable. For this reason, at the present stage the TFE membrane is, at both upper and lower surfaces, supported by a polypropylene (pp) supporter. Further, the end cap is also made of PP. Accordingly, end sealing operation is forced to be carried out at a considerably lower melting point of PP than that of TFE. In other words, such end sealing operation is dependent on the physical properties of PP, so that the superior properties of TFE disappear. It is to be noted that the melting point of TFE is 327.degree. C. at temperature, while that of PP is more or less 167.degree. C.
In order to remove the aforementioned disadvantages of the prior art, this invention has been achieved.